Calibration of medical sensors a short time before their use is desirable to help ensure that the sensor yields accurate results. Many sensors are unstable due to the materials comprising the sensors of or their design.
As an example the output signals from state-of-the-art sensors may vary due to simple changes in environment, such as temperature changes, while other sensors are subject to change simply from aging. Further, many sensors do not have a linear response over their stated range. For these sensors, calibration must be performed over a relatively small range to insure accuracy. Another example concerns the calibration of chemical sensors, such as a pH sensor, used for fetal monitoring. With such sensors, it is necessary that calibration be accomplished by measuring at least two operating or sensing points, because the operating response of such sensors is not always linear.
The calibration of medical sensors has typically been accomplished by using two separate containers, each containing a different calibration fluid, with each calibration fluid being at or near the opposite ends of the expected range to be measured. A trained technician would first record the sensor's output with the sensor submerged in the first calibrating fluid. Then the technician would move the sensor from the first container to the second container, where a second calibrating fluid would be used to produce a second calibrating point. Since the two calibrating fluids are at opposite ends of the expected range to be measured, the sensor would be accurate over that range after being calibrated. For example, if calibrating a pH sensor for use where an expected value of a pH measurement is 7.20.+-.0.20, one calibrating fluid would have a pH of 7.0 and the other calibrating fluid would have a pH of 7.40.
When using the two container method of calibration as discussed above, the problem of maintaining sterility of the sensor and the calibrating fluids during calibration and thereafter of the sensor prior to actual use thereof becomes critical. When moving the sensor from one container to the other sensor one of the calibrating fluids may easily be contaminated.
Sterility is a major concern in the use of medical sensors since any contamination of the needle and sensor may lead to further complications, caused by infection. Further, if either the sensor or the calibrating fluids become contaminated, the accuracy of the sensor or of the calibration procedure may be substantially reduced, rendering the results misleading, which may cause a doctor to make a wrong prognosis or treatment decision.
In using the two container method mentioned above, there is substantial risk that contamination will occur due to exposure of the sensor to air sensor and the calibrating fluids. In order to alleviate this problem a solution was disclosed in U.S. Pat. No. 4,689,308 to Gerhard. Gerhard discloses a method of calibrating a sensor wherein the sensor is isolated in a double container. The sensor is first calibrated in a first calibrating fluid and then the vessel containing the first calibrating fluid is released from its seal, causing the first calibrating fluid to mix with a second calibrating fluid contained within the second, outer vessel, creating the fluid in which a second calibration is made. While Gerhard is a significant improvement over previous sensor calibration procedures, the present invention further simplifies multiple point calibration, by using two separate calibrating fluids without mixing thereof, which alleviates the problem of having to measure exact amounts of each calibrating fluid to be placed in the double container and maximizes the time the sensor remains in a sterile environment.
Another requirement of any good calibration system is that the sensor must be kept hydrated at all times. The present invention not only provides a system wherein the sensor remains hydrated, but also maintains the sterility of the sensor during subsequent storage until the time of actual use of the sensor.